Method for automatically balancing the volumetric filling of cavities

ABSTRACT

The invention relates to a method for automatically balancing the volumetric filling of cavities ( 5 ), especially of multiple injection molding dies ( 1 ), which is characterized by detecting the temperature profile in the cavities ( 5 ) and averaging the same for all cavities ( 5 ).

[0001] The invention relates to a method for automatically balancing thevolumetric filling of cavities, in particular of multiple injectionmolds, and to a corresponding multiple injection mold.

[0002] For cost reasons, it is often the case that a plurality of partsare produced simultaneously each injection cycle when producing allkinds of injection moldings, for example from plastic, metal, ceramic,etc. To do this, multi-impression molds are used, the individualcavities normally being balanced out in terms of geometry and gates to asufficient extent for the filling to be as uniform as possible.

[0003] In practice, however, fluctuations in material and temperatureand resultant fluctuations in viscosity of the injection compound meanthat the filling of the individual cavities always differs and alsochanges constantly. To achieve a molding quality which is as constant aspossible, therefore, the volumetric filling of the individual cavitieshas to be permanently monitored and, if necessary, adjusted. Inprinciple, the volumetric filling can be adjusted by changing theindividual hot runner temperatures of the individual cavities.

[0004] U.S. Pat. No. 6,090,318 describes a method for controlling thehot runner heating in a multiple-cavity injection mold, in which theprofiles of the mold internal pressure in all the cavities is recordedand the times from the start of the filling operation until a suitablyselected reference pressure are measured for each cavity. By interveningin the individual heating means of the individual hot runners, thetemperature in these runners are altered in such a way that the measuredtimes for the individual cavities move closer together.

[0005] A fundamental drawback of this method is the high cost of thesystem for the sensors and amplifiers. Furthermore, the temporalpressure profile of the individual pressure curves is compared with meanvalue curves, requiring detailed process knowledge and making universaldeployment more difficult.

[0006] The present invention is based on the object of significantlysimplifying the above-mentioned method and achieving significant costsavings.

[0007] This object is achieved by the fact that the temperature profilein the cavities is determined and is made more uniform for all thecavities.

[0008] Therefore, the method demonstrates a possible way of recordingthe time offset of the individual signals, the maximum values ofindividual signals and integral values with the aid of temperaturesignals and using these values as control parameters. Multi-channeltemperature measurement systems are significantly less expensive andless sensitive than pressure measuring systems.

[0009] The main information in connection with the control of ahot-runner multi-impression mold is the temporal offset of the signals.If the rise in these signals for the individual cavities is identical,they are automatically being volumetrically filled simultaneously.

[0010] It is preferable to determine the temperature at the end of thefilling path, i.e. preferably after 95-98% of the filling path. In thisrespect too, the method according to the invention differs from theknown methods involving determination by pressure sensors. The pressurein the cavity may even rise without the cavity having been completelyfilled. However, the temperature profile at the end of the filling pathcan only be achieved if complete filling of the mold cavity also takesplace at the same time.

[0011] It is preferable for the temperature profile at the end of thefilling path to be presented to the operating staff on a displayillustrating the current temperature profile against a reference value.The reference value used is the mean temperature rise of all thecavities observed. If this shows deviations from the reference value,such deviations are clearly indicated by the fact that the curve profileis no longer presented as a straight line. A 45° straight line onlyresults if there is a balanced, controlled correspondence between theactual temperature profile and the reference value.

[0012] Further advantages, features and details of the invention willemerge from the following description of preferred exemplary embodimentsand with reference to the drawing, in which:

[0013]FIG. 1 shows a diagrammatically depicted cross section through aninjection mold;

[0014]FIG. 2 shows a diagram illustrating the pressure profile inindividual cavities of a multiple injection mold.

[0015]FIG. 1 illustrates a closed injection mold 1. It substantiallycomprises two mold plates 2 and 3, with cores 4, which in the closedposition engage in cavities 5 in the mold plate 2, projecting from themold plate 3. Cores 4 and cavities 5 together in each case form animpression 6 which, by way of example, may be filled by a moltenplastic. This plastic is passed through a hot runner 7 of a nozzle 8 andinjected into the impression 6 at a gate 9.

[0016] According to the invention, it is preferable for each cavity tobe assigned at least one temperature sensor 10. This temperature sensor10 is located at the end of the filling path of the cavity 5, preferablyat approximately 95-98% of the filling path of the cavity.

[0017] In the nozzles 8 there are heating passages 11, by means of whicha molten plastic in the hot runner 7 is held at a desired temperature.By contrast, in the mold plate 2, in the region of the cavities 5, thereare cooling passages 12, which in turn bring the plastic in theimpression 6 to a desired temperature, so that it solidifies there aftera desired period of time and the molded article can be removed from thecavity 5.

[0018] The present invention functions in the following way:

[0019] Plastic is injected into each impression 6 via the nozzle 8 orthe hot runner 7. The temperature profile in the cavity 5 is determinedat the end of the filling path by the temperature sensors 10. Inaccordance with FIG. 2a, the temperature rises when the molten plasticreaches the end of the cavity and then drops again as a result ofcooling by means of a cooling medium in the cooling passages 12.

[0020] In this context, it has been found that the temperature profilein the individual cavities 5 may differ, which is undesirable. What isin fact desirable is to make the temperature more uniform from a minimumvalue to a maximum value. This means that a mean temperature rise istaken as a reference, and the actual temperature rise of each cavity isplotted against this reference. If the temperature rise of each cavitycorresponds to the mean temperature rise, the result is a 45° straightline, as shown in FIG. 2b. By contrast, if the temperature profilesdeviate from the reference value, this results in each cavity having adifferent curve compared to the straight line in FIG. 2b.

[0021] Measuring the temperature at the end of the filling path alsosimultaneously gives an indication of whether all the impressions of amultiple mold are completely filled simultaneously. This represents afurther significant difference with respect to the known methods.

List of Reference Symbols

[0022]1 Injection mold

[0023]2 Mold plate

[0024]3 Mold plate

[0025]4 Cores

[0026]5 Cavity

[0027]6 Impression

[0028]7 Hot runner

[0029]8 Nozzle

[0030]9 Gate

[0031]10 Temperature sensor

[0032]11 Heating passage

[0033]12 Cooling passage

1. A method for automatically balancing the volumetric filling ofcavities (5), in particular of multiple injection molds (1),characterized in that the temperature profile in the cavities (5) isdetermined and is made more uniform for all the cavities (5).
 2. Themethod as claimed in claim 1, characterized in that the temperature isdetermined at the end of the filling path, preferably at 95-98% of thefilling path.
 3. The method as claimed in claim 1 or 2, characterized inthat a mean temperature rise of all the cavities (5) is taken as areference, and the deviation of the individual cavity from thisreference value is determined, and this deviation is brought into linewith the reference value by influencing the temperature of each cavity(5).
 4. The method as claimed in claim 3, characterized in that thecurrent temperature data are plotted in a time-independent manneragainst the reference data, so that optimum correspondence results in a45° straight line, or in the event of a lack of correspondence it isattempted to achieve a 45° straight line by changing the temperature. 5.A multiple injection mold having a plurality of cavities (5) forreceiving molten plastic, characterized in that each cavity (5) isassigned a temperature sensor (10).
 6. The multiple injection mold asclaimed in claim 5, characterized in that the temperature sensor (10) isassigned to the cavity (5) at the end of the filling path.
 7. Themultiple injection mold as claimed in claim 6, characterized in that thetemperature sensor (10) is arranged after 95 to 98% of the filling pathof the cavity (5).